The peritubular capillary permeability to albumin, total lymph drainage from the kidney and abnormal constituents in renal lymph related to cellular injury are studied in this project, under conditions of experimental renal disease and in spontaneously hypertensive rats. Quantitatively, the permeability times surface area product (ps) to albumin of the renal peritubular capillaries is determined as the ratio of extravascular distribution space of albumin over the mean residence time of albumin molecules in this pool. The indications are that in a very early phase of HgCl2 poisoning, the capillary ps product to albumin remains relatively constant and the change in distribution volume and residence time of albumin in the interstitium are explicable by an altered fluid balance in the extravascular compartment of the renal cortex. Exploration of consecutive changes is in progress.